[postgis-devel] Another idea to speedup raster value editing

[Bryce L Nordgren]

On Mon, Feb 6, 2012 at 5:06 PM, Pierre Racine
<Pierre.Racine at sbf.ulaval.ca>wrote:

>
> > This suggests a user visible API addition of
> >
> > ST_DumpRasterAsGeomval(rast raster, cell_pt geometry OUT,  values array
> > OUT)
>
> How is this different from ST_DumpAsPolygons()? I agree that
> ST_DumpAsPolygonValues() would have been clearer...
>
>
Well, the geometry column would contain points (cell centers), for one.
Values is also an array instead of a scalar, making this a true raster
operation instead of a single band operation.

With this call, you're working with pixels instead of the perimeter of
aggregated clumps of pixels. That's vital when you start thinking
multiband. Individual pixels can always have a unique combination of band
values. Aggregations of pixels...well, in certain special cases where the
bands are well correlated, I suppose it may work OK. But on real data,
you'd end up with one polygon containing all the pixels having the value
[12, 15] and another polygon for pixels having value [12, 22]...assuming
you put in a lot of effort to implement a multiband DumpAsPolygons.


> > ST_LoadRasterFromGeomval(rast raster, t_name text, pt_col text default
> > "cell_pt", values array)
> >
> > t_name = table name
> > pt_col   = name of column containing points
> > values = array of column names containing values to load into raster.
>
> How is this different from the planned ST_UnionToRaster()? What do you do
> when two or more points fall into the same pixel?
>

First question: it appears that ST_UnionToRaster: a] is not planned to
operate on an existing raster; b] will apply the same value to every
geometry; c] does not specify if it will even take "points", or what the
outcome might be if you provided points.

Second question: do whatever is appropriate. If you're considering the
pixels to be an area, then average the values within the pixel, or take the
max, or the min, depending on what you're interested in. Or consider the
array of geomvals to represent layers in "z order" (topmost layer wins). If
two or more polygons fall within the same pixel, then weight the average by
the area of each polygon. If you're considering the pixels to represent
point samples, interpolate (krige, IDW, etc.). (And by "you", I don't mean
the authors of the tool, I mean users.) In any case, these are all meant to
outline the array of use cases which are likely to be common, support
whatever subset is convenient.

But realize that by asking that second question, you have placed yourself
squarely in "Coverage (ISO19123)" territory. Deciding what value to return
at a point based on a set of geomvals is exactly what a coverage does. It
may be worth your while to investigate their solution to this problem
before rolling your own.
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